JP3322890B2 - Gamma offset adjustment circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma offset adjustment circuit for accurately setting an input reference DC level of a video signal with respect to a gamma correction curve of a gamma correction circuit in a video signal processing circuit of a liquid crystal TV, for example.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional gamma correction circuit. This circuit includes the non-inverting input of the current output amplifier 1 +
Is connected to the input terminal 2, the input reference power supply 3 is connected to the inverted input −, the output terminal 4 is connected to the output, and one end of the nonlinear impedance 5 is connected. The other end of the nonlinear impedance 5 is connected to the reference power supply 6. Connected.

FIG. 8 shows a specific configuration of the nonlinear impedance 5 shown in FIG. The output 4 of the current output amplifier 1 is connected to the bases and collectors of the transistors Q9 and Q10 and one end of a resistor R5. Transistor Q9
Is connected to the positive electrode of the reference power supply 11 via the resistor R6.
The emitter of the transistor Q10 is connected to the negative electrode of the reference power supply 11 and the positive electrode of the reference power supply 12 via the resistor R7. The other end of the resistor R5 is connected to the negative electrode of the reference power source 12 and the reference electrode 6.
Connect to

The input / output characteristics of the circuit shown in FIG. 7 have a gamma correction curve shown in FIG. For example, if the pedestal level of the video signal at the input terminal 2 is to be taken as the origin of the gamma correction curve, the DC level at the input terminal 2 or the DC level at the input reference potential 3 is adjusted, and the desired DC level is obtained at the output terminal 4. What should be done is.

However, when the above circuit is to be constructed in an IC, in this method, the reference potential of the gamma correction curve varies from one IC to another, and there is also an output offset current of the current output amplifier 1, so that the gamma correction can be accurately performed by the output. Since the origin of the curve cannot be known, accurate adjustment cannot be performed. As one means of this adjustment, a method of performing pedestal clamping by input has been considered. However, the pedestal level of the actual video signal may change according to the signal level due to DC transmission correction, etc.
When such a signal is input, it is not effective.

[0006]

In the conventional gamma offset adjustment circuit described above, when a pedestal clamp circuit is not used for input, the relative position of the input level with respect to the reference potential in the IC is not known. On the other hand, there is a problem that it is difficult to adjust to a desired position.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a circuit capable of accurately adjusting an input DC level to a desired position with respect to a gamma correction curve without an input pedestal clamp circuit.

[0008]

Means for Solving the Problems The present invention provides a gamma complement.
Fixed at any timing to the reference potential that determines the positive output
It has a buffer circuit that can
When the buffer circuit operates and when it does not operate
Is adjustable.

[0009]

According to the above-mentioned means, for a certain input level, the output DC level difference between when this circuit is operated and when it is not operated is read, and the level difference is adjusted so as to be set arbitrarily. Curve input D
The relationship with the C level can be appropriately determined.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which the same parts as those in FIG. 7 are denoted by the same reference numerals, and different parts will be mainly described here.

FIG. 1 differs from FIG. 7 in that a buffer circuit 7 controlled by a control signal input to a control terminal 8 is connected in parallel with the nonlinear impedance 5 in FIG. The buffer circuit 7 connects the input 9 to the connection point between the nonlinear impedance 5 and the reference power supply 6, and
Is connected to the output terminal 4.

FIG. 2 specifically shows the buffer circuit 7 of FIG. That is, transistors Q1, Q2,
Q3, mosquito Les Ntomira CM1 constitute an operational amplifier. The base of the transistor Q 1, which is the non-inverting input of the operational amplifier, is connected to the nonlinear impedance 5 and the reference power supply 6 of FIG. 1 via the input 9 of the buffer circuit 7, and the output of the operational amplifier is connected to the output terminal 5. Transistors Q4, Q
5. The current source of the operational amplifier composed of the resistors R1 and R2 connects the bases of the transistors Q4 and Q5 serving as current control inputs to the control terminal 8.

When a collector current flows through the transistors Q4 and Q5 under the control of the control terminal 8, the buffer circuit 7 changes the potential of the output terminal 4 to the input 9 even if there is an output offset of the current output amplifier 1 of FIG. Of the potential.

In FIG. 1, for example, the output terminal 4 is connected to FIG.
When a control pulse as shown in FIG. 3B, which is turned on during the pedestal period, is given to the control terminal 8 while the video input as shown in FIG. 3A is being output, the output terminal 4 is turned on as shown in FIG. Output waveform is obtained. This means that the reference potential 6 in FIG. 1 is inserted into the output when the control pulse is ON. Therefore, by adjusting the amplitude of the reference potential inserted into the output terminal 4 , the input signal DC level can be appropriately set for the gamma correction curve. By the way, if you want to adjust the pedestal of the input signal to the origin of the gamma correction curve,
What is necessary is just to adjust so that a step does not appear even if the reference potential is inserted.

In the above-described embodiment, the input-referred DC offset generated in the gamma correction circuit can be easily checked with a relatively simple configuration, so that the input DC level can be set optimally.

FIG. 4 shows another embodiment of the present invention. In this embodiment, the input and output of the buffer circuit 7 in FIG. 1 are connected in reverse, and the output 10 of the buffer circuit 7 connected in parallel is connected to the connection point of the nonlinear impedance 5 and the reference power supply 6 via the resistor R3. supplies the output of the buffer circuit 7 to the differential amplifier 11, its output
Are connected to the output terminal 4.

In this embodiment, the output waveform of the current output amplifier 1 is as shown in FIG. When the control pulse from the control terminal 8 activates the buffer circuit 7, this output becomes as shown in FIG. 5B. As a result, the levels of the waveforms (a) and (b) are subtracted by the differential amplifier 10, and the final output is as shown in FIG. In this embodiment, since the input of the buffer circuit 7 is connected to the signal path, the effect of the f characteristic deterioration due to this is negligible, which is extremely advantageous for widening the band.

The configuration of the buffer circuit 7 of the embodiment described here is not particularly limited to FIG. For example,
A similar operational amplifier as in FIG. 2, rather I as long as it can stop its function due to OFF current source may be a switching circuit as shown in FIG. The circuit of FIG. 6 interconnects the bases of transistors Q6, Q7 and the collector of transistor Q8,
The collector of Q7, the emitter of Q7 is connected to input 9, the emitter of transistor Q6, the collector of Q7 is connected to output terminal 4, the emitter of transistor Q8 is grounded via resistor R4, and the base is connected to control terminal 8. ing.

In this circuit, when the collector current of the transistor Q8 flows under the control of the control terminal 8, the transistors Q6 and Q7 enter the saturation mode, the impedance between the switch input 9 and the output terminal 4 becomes low, and the circuit becomes conductive. On the other hand, when the transistor Q8 is cut off, the area between the switch input 9 and the output terminal 4 is open. The point is that the DC level of the output terminal 4 may be equivalent to the DC level of the input 9 equivalently depending on the state of the control input.

In this description, the reference level is set to an appropriate position with respect to the pedestal level of the video signal. However, the reference level is not particularly limited.

[0021]

As described above, according to the gamma offset adjusting circuit of the present invention, there is an effect that gamma correction can be appropriately performed on an input signal with a very simple configuration.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram specifically showing a main part of FIG.

FIG. 3 is a waveform diagram of a main part of FIG. 1;

FIG. 4 is a circuit diagram showing another embodiment of the present invention.

FIG. 5 is a waveform diagram of a main part of FIG.

FIG. 6 is a circuit diagram specifically showing a main part of FIG. 4 ;

FIG. 7 is a circuit diagram of a conventional circuit.

FIG. 8 is a circuit diagram specifically showing a main part of FIG. 6;

9 is an input / output characteristic diagram of FIG.

[Explanation of symbols]

 1 ... current output amplifier 2 ... input terminal 4 ... output terminal 5 ... nonlinear impedance 6 ... reference power supply 7 ... buffer circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/14-5/217

Claims (1)

(57) [Claims] [Claim 1] A reference DC level can be adjusted in the DC offset of the signal including the period with and the signal a gamma offset adjustment circuit for performing gamma correction, the preceding item relaxin as one differential input said A differential amplifier circuit having a variable DC voltage source for adjusting a DC offset as the other of the differential inputs; a non-linear impedance circuit driven by a current output of the differential amplifier circuit; a reference of a voltage output generated by the driving A reference voltage source to which one end of the nonlinear impedance circuit is connected , and the other end of the nonlinear impedance circuit to the output and
The voltage output of the reference voltage source is connected to the input or
Indicates that the other end of the nonlinear impedance circuit is
Serial voltage output of the reference voltage source is connected to the output, before the input only in the period in which the preceding item relaxin has a pre-Symbol reference dc level
A buffer circuit for transmitting the signal to the output.
Output waveform of the buffer circuit during the period having
Is adjusted based on the reference, and the non-linear
A gamma offset adjustment circuit, which outputs a voltage output generated in an impedance circuit .